This invention relates to methods of indicating the release of one or more plugs in a pipe or tube system of an oil or gas well. Such a system includes a drill, completion or production string (any such system is referred to in this specification and the claims simply as "tube system" or specifically "plug release tube system"). Particular utility is with regard to plugs used in a cementing operation in an oil or gas well, especially a subsea well.
Cement is used in oil or gas wells for various purposes. One purpose is to secure a tubular string (e.g., a casing or a liner) in the well bore. This is typically done by pumping cement down the tubular string and forcing it back up an annular space between the outside of the string and the inside of the well bore or of a larger diameter string in which the first-mentioned string is disposed.
To separate the cement slurry from drilling mud typically in the well when the cementing operation begins, a bottom cementing plug is placed in line and pumped down the string by the force of the following cement slurry. This bottom plug serves to minimize contamination of the cement as it is pumped down the tubular string. The bottom plug also wipes any accumulated mud film from the inner diameter of the string and pushes it ahead.
To separate a following displacing fluid used to push the cement slurry out the tubular string and up the annular space, a top cementing plug is placed in line and pushed down the string by the displacing fluid. This top plug follows the cement and wipes any accumulated cement film from the inner diameter of the tubular string. It also prevents or reduces any contamination of the cement by the displacing fluid.
In wells drilled on land, surface-mounted plug containers are used in many cementing jobs to release the cementing plugs at the proper time. Normal job operations will have the bottom cementing plug loaded into the plug container prior to pumping cement. The top cementing plug will typically be loaded after the bottom plug is released. If well conditions dictate, two plug containers or a double plug container may be used to release both cementing plugs when desired without opening the plug container.
Subsea (ocean floor) completions are different from the aforementioned land-based cementing operations in that the cementing plugs used for separating the fluids are preferably located in the tubular string below the ocean floor. This is preferred because these plugs have a diameter large enough to wipe the inner diameter of the tubular string extending below the ocean floor, and this tubular string (and thus each plug) typically has a larger diameter than need be used for connecting this string with the equipment on the rig at the ocean's surface. Thus, the cement slurry is preferably pumped from the surface through a string of drill pipe smaller than the string being cemented, which smaller string extends between the surface rig and the downhole string to be cemented. This creates the need for a second type of plug container that houses elements, which may broadly be called "plugs" also, which are of smaller diameter to permit these plugs to pass through the narrower connecting string and into the downhole cementing plugs. A system using this technique is the Halliburton Energy Services'sub-surface release system ("SSR Cementing Plug Method"). This system provides a means of wiping different pipe sizes; therefore, smaller diameter drill pipe can be used as described instead of the larger diameter casing that otherwise would be run between the rig floor and the ocean floor.
This prior art subsea release system will be briefly explained with reference to FIGS. 1-3. These drawings schematically illustrate the sequence of operation.
FIG. 1 shows bottom and top cementing plugs 2, 4, respectively, installed at the top of casing 6 (i.e., the tubular string in the subsea well bore) prior to beginning the actual cementing operation. A set of releasing pins attaches the bottom cementing plug 2 to the top cementing plug 4.
A weighted plastic or bronze ball 8 housed in a surface plug container 10 is dropped through connecting drill pipe 12 ahead of the cement slurry. The drill pipe 12 connects the casing 6 in the subsea well bore and the plug container 10 at the surface. The ball 8 passes through a wider axial channel of the top plug 4 and lands on a seat of the bottom plug 2. A differential pressure applied through the drill pipe 12 from the surface separates the thus sealed bottom plug 2 from the top plug 4.
FIG. 2 illustrates how the bottom plug 2 has been discharged from the top plug 4 and seated on a float collar 14 (or float shoe). At this point, a small increase in pressure exposes port holes in the plug 2 so that the cement slurry can be pumped around the bottom plug releasing ball 8.
A collet releasing mechanism holds the top plug 4 in place and permits circulation through the top cementing plug 4 at normal displacement rates prior to release of the top plug 4. To release the top cementing plug 4, a top releasing plug 16 from the surface plug container 10 is pumped down the drill pipe 12 behind the slurry and into the top cementing plug 4 where it latches and seals therewith. An applied pressure shears releasing pins to enable the top plug 4 to move down the casing 6.
As shown in FIG. 3, the top cementing plug 4 lands on the bottom cementing plug 2 to shut off flow in conventional manner.
It is desirable to know that a plug used in the operation described above has properly released. If the ball 8 for the bottom plug 2 is released and allowed to free fall, a pressure change sensed at the surface can be used to indicate seating of the ball and then to indicate release upon pumping the cement or other fluid into the well after the free fall has ended. With offshore drilling moving into deeper waters, however, it is undesirable to allow the weighted ball for the bottom plug release to free fall to the ball seat. Wells drilled in waters over 6,000 feet deep, for example, would require approximately thirty minutes for the weighted ball to reach the ball seat. To avoid this delay, the cement slurry is pumped immediately after the weighted ball is released, but this practice usually does not show a pressure indication that the bottom plug has released. Thus, to allow for the expedited delivery of the cement by pumping it immediately behind the ball 8 and yet obtain an indication of plug release in the well, there is the need for a method to indicate plug release without relying on a pressure indication. This need also exists with regard to surface mounted plugs. There is also the need for the indication produced by any such method to be readily communicated.